Tidal Energy Overview

Tidal energy differs from all other energy sources in that the energy is extracted from the potential and kinetic energies of the earth-moon-sun system. The well known ocean tides result from this interaction, producing variations in ocean water levels along the shores of all continents. As the water level fluctuates twice daily through this range, it alternately fills and empties natural basins along the shoreline, suggesting that the currents flowing in and out of these basins could be used to drive water turbines connected to generators and thus to produce electricity. The higher the tides, the more electricity can be generated from a given site, and the lower the cost of electricity produced. The technology employing this energy source is very similar to that of low head hydropower. 

POTENTIAL : 

World-wide, approximately 3000 GW of energy is continuously available from the action of tides. Experts estimated that only 2% (60 GW), what is about 50 times less than the world’s potential of hydroelectric power capacity, can potentially be recovered from tides for electricity generation. Currently, only in places with large tidal range (greater than 5 meters) can tidal power be extracted economically.
In some places of the world tidal energy is quite attractive. For coastal areas, usually at the entrances to large estuaries, resonance can occur, leading to far greater than average tidal ranges which could relatively conveniently be blocked off. Such circumstances are found e.g. in Canada, with a mean tidal range of 10,8 metres or in the Severn Estuary in Britain with a mean range of 8,8 metres, making large scale projects at both these locations economical. 

DEVELOPMENT : 

Tidal power plant at La Rance River has turbines that can also serve as pumps; thus, the installation can function as a pumped hydro storage facility to even out the loads on a large electricity generating and distribution system. In this way water pumped into the basin during times of low power demand increases the head on the turbines at other times. Tidal range there is up to 13,4 meters. The dam’s width is 760 meters. At high tide, the dam traps Atlantic waters in the bay. At low tide, the water flows back to the sea. En route, it passes through 24 turbines connected to generators that produce 240 megawatts of power. This provides enough electricity for a city of 300.000. In 1997, they began installing turbines that can spin on both the incoming and outgoing tides.  

TECHNOLOGY : 

Tidal power is a proven technology: it has been used for centuries in small mill-type applications where natural conditions make it possible. Tidal energy can be converted into electrical energy in several ways. Conventional systems such as barrages (or low dams) store water in inlets from high tides for release through hydraulic turbines during lower tides. The newest technology which converts tidal or coastal currents to power seems to be very promising because it is less environmentally destructive.  

The usual technique (referred to as “barrage” technology) is to dam a tidally-effected estuary or inlet, allowing the tidal flow to build up on the ocean side of the dam and then generating power during the few hour high tide period. In this way it is functioning in La Rance. After the water level reaches maximum high tide, gate valves are closed and the water is impounded and awaits low tide when it is released and produces power. The gates can be opened or closed in sequence with the tides permitting water flow only when there is sufficient head to power the turbines. The basic technology of power production is similar to that for low head hydro power plants what means that the head drives the water through the turbine generators. The main difference, apart from the salt water environment, is that the turbines in tidal barrages have to deal with regularly varying heads of water. The turbines are designed so that the flow of water both into and out of the basin produces electricity. Because of the intermittent nature of this flow, the effective duty factor of such an installation is less than 100%. A tidal power station produces only about one third as much electrical energy as would a hydroelectric power plant of the same peak capacity operating continuously. Tidal barrages are effectively fences which completely block a estuary channel.  

ENVIRONMENT : 

The barrage does not easily scale up to modern commercial levels of output capacity. By increasing the size of the pond one increases the four major negative environmental impacts of the barrage technology: navigation is blocked, fish migration is impeded and fish are killed by passing through the turbines, the location and nature of the intertidal zone are changed, and the tidal regime is changed downstream. Reduced tidal range would destroy much of the habitat used by wading birds, fish (such as salmon) would be unable to travel upstream to breed, and sediment trapped behind the barrage could quickly reduce the volume of the estuary. It seems that while there are few environmental impacts associated with a smaller tidal facility, (i.e., no siltation, no negative impacts to water tables, fisheries or fish migration), larger operations could potentially limit fish and mammal passage and change tidal ranges, thereby effecting salt water intrusion into local tributary streams and impacting salmon spawning.  

TIDAL TURBINES  : 

By the early 1990s, interest in estuarine-derived tidal power had declined, and scientists and engineers began to look at the potential of coastal currents which can be harnessed by tidal turbines. Instead of using costly barrages and low head turbines located in estuaries, it may be possible to harness the kinetic energy of the tides in fast tidal currents or streams at suitable sites, using relatively simple techniques - tidal turbines. As tides ebb and flow, currents are often generated in coastal waters (quite often in areas far-removed from bays and estuaries). In many places the shape of the seabed forces water to flow through narrow channels, or around headlands (much like the wind howls through narrow valleys and around hills). However, sea water has a much higher density than air (832 times). Thus, currents running at velocities of 5 - 8 knots (9,25 km/h – 16,7 km/h) have the same energy potential as a windmill site with windspeeds of 390 km/hr! In addition, unlike the wind rushing through a valley or over hilltops, tidally-generated coastal currents are predictable. The tide comes in and out every twelve hours, resulting in currents which reach peak velocity four times every day.  

Tidal turbines are the chief competition to the tidal barrages but the idea is as yet relatively underdeveloped. Looking like an underwater wind turbine they offer a number of advantages over the tidal barrages. They are less disruptive to wildlife, allow small boats to continue to use the area, and have much lower material requirements than the dam. Tidal turbines function well where coastal currents run at 2-3 m/s (slower currents tend to be uneconomic while larger ones put a lot of stress on the equipment). In such currents a turbine 20m in diameter will generate as much energy as a 60m diameter windmill. The advantages of the tidal turbine is that it is neither seen, nor heard. The whole assembly (apart from the transformer) is below the waterline.  

There are many sites around the world where tidal turbines would be effective. Coastal currents are strongest at the margins of the worlds larger oceans. A review of likely tidal power sites in the late 1980s estimated the energy resource was in excess of 330.000 MW. South East Asia is one area where it is likely such currents could be exploited for energy. In particular, the Chinese and Japanese coasts, and the large number of straits between the islands of the Philippines are suitable for development of power generation from coastal currents. In all of these regions underwater turbine farms can be developed. The ideal site is close to shore, in water depths of about 30m where at the best sites currents could generate more than 10 megawatts of energy per square kilometre. The European Union has already identified 106 sites which would be suitable for the turbines, 42 of them around the UK. The first tidal turbines will be deployed off the Southwest coast of England. It will be 12-15 m in diameter, and is expected to generate 300 kW (enough to power a small village). It is estimated that the cost of energy from these early turbines will be USD  0,10/kWh. This is more expensive than conventional sources of energy (coal, gas), but significantly lower than what many island communities already pay for energy. As the technology matures further, prices will probably continue to drop.  

                                                                                  

How it works:   Tidal Barrages : 

These work rather like a hydro-electric scheme, except that the dam is much bigger. 

A huge dam (called a "barrage") is built across a river estuary. When the tide goes in and out, the water flows through tunnels in the dam. 

The ebb and flow of the tides can be used to turn a turbine, or it can be used to push air through a pipe, which then turns a turbine. Large lock gates, like the ones used on canals, allow ships to pass.

If one was built across the Severn Estuary, the tides at Weston-super-Mare would not go out nearly as far - there'd be water to play in for most of the time. 

But the Severn Estuary carries sewage and other wastes from many places (e.g. Bristol & Gloucester) out to sea. A tidal barrage would mean that this stuff would hang around Weston-super-Mare an awful lot longer!

Also, if you're one of the 80,000+ birds that feeds on the exposed mud flats when the tide goes out, then you have a problem, because the tide won't be going out properly any more.

Advantages : 

• Once you've built it, tidal power is free.
  
  
• It produces no greenhouse gases or other waste.
  
  
• It needs no fuel.
  
  
• It produces electricity reliably.
  
  
• Not expensive to maintain.
  
  
• Tides are totally predictable.
  
  
• Offshore turbines and vertical-axis turbines are not ruinously expensive to build and do not have a large environmental impact.

Disadvantages : 

• A barrage across an estuary is very expensive to build, and affects a very wide area - the environment is changed for many miles upstream and downstream. Many birds rely on the tide uncovering the mud flats so that they can feed. Fish can't migrate, unless "fish ladders" are installed.
  
  
• Only provides power for around 10 hours each day, when the tide is actually moving in or out.
  
   

Tidal barrage, Rance Estuary, France ( video )